Application of MBS (Multicast Broadcast Service) is studied or started in various wireless communication systems (for example, 3GPP, WiMAX, and the like). The contents in MBS include streaming distribution or the like, and further higher capacity as well as higher quality is required in the future.
As a method for improving quality of MBS, first, there is an application of link adaptation based on feedback (FB) information from a reception side. As shown in FIG. 1, a terminal (MS) receives packet data of MBS (hereinafter “MBS packet data”) transmitted from a base station (BS), determines whether or not the reception of the receiving packet is successfully performed, and feeds back information of reception success/failure determination result (ACK/NACK information) to the base station.
The BS variably controls a coding rate, a M-ary modulation value, a transmission power, and the like when transmitting an MBS packet next time and after that on the basis of a plurality of FB information pieces transmitted from a plurality of MSs in a communication area. The BS appropriately performs control (specifically, link adaptation) so that MBS in the communication area has a required quality. By this means, it is possible to secure a required quality of MBS in the communication area and suppress allocation of resources generating excessive quality, and as a result, the frequency use efficiency can be improved.
Here, the reception success/failure determination performed by MS is performed by using, for example, CRC (Cyclic Redundancy Check) added to each transport block (TB) in the physical layer. If the reception success/failure determination result of an MBS packet is reception OK (that is, a case in which ACK determination is made), ACK information is transmitted from MS to BS. On the other hand, if the reception success/failure determination result is reception NG (that is, a case in which NACK determination is made), NACK information is transmitted. The ACK information and the NACK information are transmitted through a predetermined common or individual uplink channel.
When such FB information is used, if there are a large number of terminals in a cell, the amount of FB information is huge, and thereby there is a problem that the frequency use efficiency in the uplink is degraded. To solve the problem, a method for reducing the amount of FB information is proposed. For example, as shown in FIG. 2, the amount of FB can be reduced by feeding back FB information only when NACK determination is made (hereinafter may be simply referred to as “NACK FB”). Further, a method is proposed in which a certain condition (FB condition) is further set and NACK information is fed back only when the condition is satisfied even when NACK determination is made. For example, first, there is a method in which terminals in a cell are divided into a plurality of terminal groups and a limited time window is set for each terminal group to transmit FB information. Second, there is a method for providing a call probability, that is, a method in which each terminal uses random numbers and when a random number satisfies a predetermined condition, feedback can be performed.
As a second method for improving quality of MBS, there is a method in which error correction code in the MAC layer (hereinafter may be simply referred to as “MAC FEC”) is introduced (for example, see NPL 1, PTL 1, and PTL 2). In this method, in addition to error correction coding processing in the PHY layer, error correction coding processing is also performed in the MAC layer after interleave processing is performed on a plurality of TBs in the PHY layer. Specifically, dual coding processing over a plurality of TBs in the PHY layer is performed. By this means, a reception side MS can correct an error in TB in the PHY layer (hereinafter may be simply referred to as “PHY TB”) by performing error correction decoding processing in the MAC layer. As a result, reception quality in MS is improved. A coder that performs error correction coding processing in the MAC layer may be referred to as inner coder, and a coder that performs error correction coding processing in the PHY layer may be referred to as outer coder.